The invention relates to a high stability printing liquid, intended more particularly for printing by ink jet, requiring the use of a fluid printing liquid which is stable over a long period of time.
In certain applications of printing liquids of this type and particularly in the case of printing by ink jet, it is necessary to pass the ink through small diameter orifices, for example having a diameter of a few dozen micrometers, in order to obtain by breaking up of the jet ink droplets which, when projected onto a substrate make it possible to obtain spots with a diameter of approximately 100 to 300 micrometers. This is in particular necessary for obtaining a high density of points, characterized by the number of points per unit of length, which is called the printing resolution.
In general, there are problems regarding the moisture resistance of the printing when using hydrosoluble dyes, many of them containing metallic-ferrous groups. Thus, preference is given to the use of dyes which are soluble in an organic medium.
The use of the small diameter orifices referred to hereinbefore is not in itself sufficient for obtaining the sought high resolutions. In addition, the geometrical arrangement of the orifices must make it possible to obtain the sought resolutions over the complete surface to be printed. The small dimensions of the orifices and the large number of points sought generally make it difficult to obtain such geometrical arrangements, when each orifice only makes it possible to print a single point. It is then often advantageous to be able to print several points with each orifice. This can be obtained by moving the printing substrate and/or the orifices and by imposing different paths or trajectories on the drops issuing from the same orifice.
A convenient way to impose different paths or trajectories on the drops consists of a transfer of electrical charges thereto and a deflection of these charged drops by an electrical field. The electrical charge transfer can only be obtained if the printing liquid has an adequate electrical conductivity. This is particularly the case when the electric charge transfer time is only a few microseconds.
A printing liquid, which is often called ink in the remainder of the description, must be stable over a period of time and must be usable at any time for forming jets of droplets by passing through orifices with a diameter of only 5 to 100 micrometers. This generally eliminates the possibility of using coloured pigments which easily form aggragates of size sufficient to bring about a partial or total clogging of the orifices. Such a liquid must also lead to an adequate electrical conductivity being obtained to permit the transfer of an electrical charge to the droplets at the time when they form by separation from the jet. Moreover, the printing must dry rapidly and it must have an adequate resistance to moisture and friction. Finally, this liquid or ink must lead on the one hand to obtaining an optimum optical contrast, requiring the use of dye quantities which may, for example, be between 5 and 20% by weight of the total weight, and on the other hand to a limitation of the spread and diffusion of the dye on and/or into the substrate to obtain clearly defined and reproducible dimensions, geometry and optical contrast.
These problems are difficult to solve, particularly as these requirements are often confronted by other contradictory requirements. Thus, as the conductivity must be high it is necessary to use relatively large amounts of water to ensure a good dissociation of the electrolyte, which makes it possible to adjust this conductivity. However, organosoluble metalliferrous dyes (which alone are able to give an adequate optical contrast) do not tolerate the addition of water and over a longer or shorter term there is a danger of flocculation.